1. Technical Field
This invention relates to measurement devices for measuring groundwater levels, and more particularly to a measurement device including a sensor that measures a groundwater level and determines when the groundwater level falls below a given threshold.
2. Background Art
Measurement devices are commonly used to measure the water level in well bores. A typical method of monitoring water level drawdown is to lower a sensor into the well bore, attached to a flexible element such as a measuring tape. The sensor senses the conductivity through water and generates a signal when it makes contact with water. That signal is carried through a wire in the tape to an indicator device such as a light or an alarm, such that when the sensor makes contact with the water, the user is informed that the groundwater level has been reached.
A common way to sample groundwater from a well bore is to draw the groundwater out of the ground using a pump. It is often important in sampling groundwater with a pump to avoid pumping the water level in the well bore down to excessive drawdown levels. Otherwise, sample integrity can be compromised. In typical operation, the static water level is first determined using the sensor as above. The user then lowers the sensor to the level of maximum desired drawdown, chosen based on pumping conditions and pumping purpose, and begins pumping. When the water level drops below the maximum desired drawdown level, the sensor no longer senses conductivity and ceases to alert the user that conductivity is detected. The user must then manually adjust pump operation.
One drawback with this approach to monitoring drawdown is that the indicator device must be audibly monitored constantly throughout the pumping process to verify that the water level is continually above the maximum drawdown level. Additionally, the alarm is being emitted virtually continuously, since it is present during the normal pumping operation. It is an object of the present invention to alert the user only when the water level is below the maximum drawdown level, rather than above this level.
Another problem in sampling groundwater is when the water level frequently drops below the maximum drawdown level due to an excessive pumping rate and a lapse in the operator""s monitoring of water level and/or control of flow rate. Proper operation requires the operator to make frequent adjustments of the sampling pump in response to frequent observation of water level changes in the well bore. It is therefore another object of the present invention to automate the adjustment of the pump as necessary to prevent the maximum desired drawdown level from being exceeded.
The above and other objects are provided by a method and apparatus for measuring the groundwater levels in accordance with preferred embodiments of the present invention. The present invention provides a method and apparatus for measuring the level of a fluid, such as water, in a well bore. In a preferred embodiment, a user first determines the static water level in the well bore by lowering a sensor into the well bore. The sensor senses when it makes contact with water, so that the user is made aware when the sensor has reached the static water level. The user then lowers the sensor further to a maximum desired drawdown level. The sensor is then reconfigured to sense when it loses contact with the water. The sensor is then monitored to determine if the fluid has dropped below the maximum drawdown level.
In a preferred embodiment of the present invention, the water level is determined using a sensor attached to a flexible element such as a measuring tape. The tape is marked in feet (or any other desired unit of measurement) such that the user can determine the depth of the sensor in the well bore. The tape includes a wire for carrying an electrical signal from the sensor to an alarm interface including an alarm. The sensor emits a signal when it makes contact with water. In one preferred embodiment, the signal is an electrical signal which is carried via the wire to the alarm interface. In a preferred embodiment, the alarm includes an audible alarm and/or a visible light.
The sensor is lowered into the well bore until the sensor comes in contact with the water at the static water level, at which time it sends a first signal to the alarm indicating to the user that the static water level has been reached. The user then lowers the sensor to the maximum drawdown level. The user then actuates a reverse response switch, which causes the alarm to activate when the sensor is no longer in contact with the water instead of when it is in contact with the water.
Another preferred embodiment of the present invention includes controlling a pump in response to the second signal. The pump is used to draw the water out of the well bore at a first pumping rate. In doing so, the pump can cause the water level to temporarily drop. When the sensor senses that the water level has dropped below the maximum drawdown level, the pumping rate is adjusted to a second pumping rate lower than the first pumping rate. In a preferred embodiment, the user adjusts the pump when the alarm engages, indicating that the water level has dropped below the maximum drawdown level. In another preferred embodiment, the alarm signal is carried by a cable to a pump controller. The pump controller automatically adjusts the pumping rate to the second (i.e. lower) pumping rate in response to the alarm signal indicating that the water level has dropped below the maximum drawdown level.